According to a new study, men have a “fatherhood gene” which determines if they are more likely to have sons or daughters. The study says that men with many brothers are more likely to father sons while those with many sisters are more likely to have daughters.  

The research, published in the journal Evolutionary Biology, involved a study of 927 family trees containing information on 556,387 people from North America and Europe going back as far as 1600.

Corry Gellatly, the study leader at the University of Newcastle, said, "The gene that is passed on from both parents, which causes some men to have more sons and some to have more daughters, may explain why we see the number of men and women roughly balanced in a population."

He added, "If there are too many males in the population, for example, females will more easily find a mate, so men who have more daughters will pass on more of their genes, causing more females to be born in later generations." Gellatly said the system would tend to balance out gender numbers.

The gene the scientists claim is heritable both a man's parents can be found in one of three variations and this affects the number of sperm carrying male or female chromosomes. The men with a version of the gene known as “mm” produce more sperm with the Y chromosome and are more likely to have sons while those with the “mf” variant produce an equal number of sperm with the female X and male Y chromosome which gives him a 50/50 chance of having a son or daughter. In the third variant of “ff” gene more production of the X sperm resulted in the likelihood of having more daughters.

Although the existence of three types of fatherhood gene is suggested, the gene itself remains undiscovered.